How Pygmy People Got Their Short Stature

Below:

Next story in Science

Pygmy traits independently evolved many times among different
peoples around the world, because shorter heights may have helped
them live in rainforests, researchers say.

The small body sizes known as pygmy
traits are seen worldwide, limited to peoples who
traditionally hunted and gathered food in tropical rainforests,
such as in Central Africa and Southeast Asia. These small
statures apparently developed independently in these populations,
an example of convergent evolution, much as fish and dolphins
both evolved streamlined bodies to better swim in their watery
worlds.

Scientists have suggested that small body size might confer a
number of evolutionary benefits for life in rainforests. For
instance, while tropical
rainforests are the most complex and diverse ecosystems on
land, home to half of all living species on the planet, "there is
actually not that much food for humans," said lead study author
George Perry, an anthropologist at Pennsylvania State University
in University Park. Small body sizes, therefore, may have evolved
because they require fewer calories. [ The Awa:
Faces of a Threatened Tribe (Photos) ]

In addition, taller individuals have more difficulty moving
through the dense vegetation of tropical rainforests. Moreover,
these forests are hot, and their humidity makes it difficult for
sweat to evaporate and cool people down. Since small bodies
generate less heat during activity, they could survive more
easily.

However, it remained uncertain whether pygmy traits actually
confer evolutionary advantages in tropical rainforests. To find
out, Perry and his colleagues analyzed the genomes of 169 pygmy
members of the Batwa people,
rainforest hunter-gatherers from Uganda in east central
Africa.

When the researchers compared Batwa DNA with the genomes of 61
members of the neighboring agricultural Bakiga population, they
found the Batwa had genetic variations linked to stature and
growth hormone. Batwa stature was also linked with how much
Bakiga ancestry the individuals had — the less Bakiga ancestry
they had, the more likely Batwa had pygmy statures — suggesting a
genetic component to Batwa stature, rather than an environmental
influence.

Additionally, the researchers discovered that the Batwa genetic
variations linked to stature and growth hormone evolved quickly
compared with the normal rate of mutation in the
human genome. This suggests these genetic variations are
somehow adaptive, proving beneficial to the Batwa.

The scientists also investigated the genomes of 74 pygmies from
Baka rainforest hunter-gatherers who lived in Cameroon and Gabon
in west central Africa, comparing this DNA with genomes from 73
members of the pygmies' agricultural neighbors. Researchers found
pygmy traits apparently had different genetic roots in the Baka
pygmies than in the Batwa pygmies, suggesting convergent
evolution.

Perry cautioned that the number of Batwa individuals studied
would probably not meet the standard or minimum for genetic
analyses of traits that depend on many genes, such as height.
However, "in the entire Batwa population, there are not as many
individuals as are in many of these studies," Perry said. The
scientists say the fact that the genetic variations they analyzed
are linked with growth hormones and stature in other groups
suggests at least some of these mutations underlie pygmy traits.

In the future, Perry and his colleagues plan to investigate other
ways besides stature in which rainforest hunter-gatherer
populations have adapted to their environment. "In addition, I
would like to expand these studies to rainforest hunter-gatherer
populations in Southeast Asia, in order to study the extent to
which any such adaptations like height have also evolved
convergently across continents," Perry said.

The scientists detailed their findings online Aug. 18 in the
journal Proceedings of the National Academy of Sciences.